Patients with hypertrophic cardiomyopathy (HCM), with or without obstruction to left ventricular outflow, usually have elevated ejection fraction at rest. Therefore, inotropic therapy is thought to be of little use. However, in patients with other diseases, left ventricular function often is abnormal only during exercise. Therefore, we used radionuclide cineangiography during exercise to assess left ventricular function and to determine whether depressed function might be induced by stress. We found that although ejection fraction at rest is above normal in patients with HCM whether or not obstruction is present, many patients without obstruction manifest depressed ejection fraction during exercise. This raises the possibility that symptomatic patients without obstruction might benefit from inotropic therapy to suppport systolic function during exertion.